Why do we assume that the image distance is equal to the far point/near point of a myopic/hypermetropic eye respectively, while trying to find the focal length of the corrective lens? Aren't the far point and near point object distances?
1 Answer
Avanika, you are quite right here. The far point is of course an object distance (and so is the near point).
Optometricians use the far point to get an idea of the corrective lens that's necessary.
Example:
My far point is about 25 cm away and I can't see really sharp beyond that. The strength of my eye-lens is too great (or the focal length is too short, which means the same thing).
The relation between focal length and strength is:
Strength (in diopters) = 1 divided by focal length (in meters)
Or:
The effect is as if a magnifying glass was 'built in' with a focal length of 25 cm (plus). You can try this yourself by holding a magnifying glass in front of your eyes and see where your 'far point' is. You will see this will be the same as the focal length of your magnifying glass.
If you add a negative (concave) lens to a positive (convex) lens, the negative lens will cancel out (part of) the positive lens, and if they have the same strength, they will cancel out completely
The optometrician will give me a negative lens of the same (opposite) strength (25 cm focal length in the example) to counteract my "built-in" positive lens.
Since
